As a hydraulic control device for a power shovel, one in which first to third circuit systems are respectively connected to first to third pumps and oil discharged by the third pump is caused to join the first and second circuit systems as necessary is known.
A control circuit disclosed in JP1998-88627A is configured so that oil discharged by a third pump is supplied to a boom cylinder when only a boom switching valve provided in a first circuit system is switched, the oil discharged by the third pump is supplied to an arm cylinder when only an arm switching valve is switched, and the oil discharged by the third pump is preferentially supplied to the arm cylinder when the boom switching valve and the arm switching valve are simultaneously switched.
Specifically, the control circuit described above includes a hydraulic accelerating valve for preferentially supplying oil discharged by the third pump to the arm cylinder. The hydraulic accelerating valve includes two pilot chambers to which a pilot pressure of the boom switching valve and a pilot pressure of the arm switching valve are respectively guided, and a spring that applies a biasing force in the same direction as that of the pilot pressure of the arm switching valve.
The hydraulic accelerating valve switches so as to supply the oil discharged by the third pump to the boom cylinder in a case where the pilot pressure of the boom switching valve overcomes the biasing force of the spring when only the pilot pressure of the boom switching valve is applied. The hydraulic accelerating valve switches so as to supply the oil discharged by the third pump to the arm cylinder by means of the pilot pressure of the arm switching valve and the biasing force of the spring when only the pilot pressure of the arm switching valve is applied. Further, the hydraulic accelerating valve switches so as to supply the oil discharged by the third pump to the arm cylinder in a case where a resultant force of the pilot pressure of the arm switching valve and the biasing force of the spring overcomes the pilot pressure of the boom switching valve when the pilot pressures of both the boom switching valve and the arm switching valve are applied.